Suboptimal prescribing practices may result in errors in the outpatient setting. One promising technology that has emerged is e-prescribing, a technology that allows prescribers to write prescriptions electronically. Proponents claim that this technology will result in enhanced formulary compliance, cost savings, and improvements in patient safety. The proposed study will take advantage of a large natural experiment based on a collaborative statewide rollout of e-prescribing in Massachusetts by Tufts Health Plan (Tufts HP) and Blue Cross Blue Shield of Massachusetts (BCBSMA), and ZixCorporation. PocketScript/tm software from the ZixCorporation is an e-prescribing system that enables physicians to electronically write and fax prescriptions to the pharmacy, and identifies possible drug interactions, supplies formulary information including insurance tiers, and allows prescribers to view patient drug history. To examine whether e-prescribing results in improvements in patient safety and reductions in cost, we will conduct a pre-post study with concurrent controls to evaluate the effects of e-prescribing on poor apparent prescribing practices (potential drug-drug interactions (DDIs) and dosing problems), formulary compliance, and selected patient outcomes (ED visits and hospitalization for chronically ill patients). To perform this research, Tufts HP and BCBSMA have agreed to provide us with all pharmacy and medical claims for 10 months of 2004 and will indicate which prescriptions were written electronically. We hypothesize that e-prescribing will result in measurable impacts on poor prescribing practices, utilization, and cost. If so, then e-prescribing will be shown to be a valuable weapon in the arsenal of physicians who wish to improve patient safety. However, our study will be important even if we find no impact on patient safety, if we are also able to show that e-prescribing can reduce costs and increase use of generic medications without adversely affecting outcomes among selected groups of patients.